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CB radio antenna question(s).... Answered

I have a UHF CB radio in the car that is now quite outdated.
Waiting for a 80CH replacement to arrive so the old one can be used as a fixed station in the house.

Recently I made a new straight whip for the car as the fibreglass one snapped off.
Now the problem I am facing is a total between reality and computer simulation.
Here is what I did:
Started with a straight steel wire on the elevated mounting - this spring mount is supposed to provide a ground plane as it was originally for a ground independent antenna.
Simple math tells that a 1/2 Lambda whip should be around 33cm long, so I started with 40 and used my truted SWR meter.
As it turned out my antenna was too short !?
Tried again with a longer wire and got a good SWR reading of 1.09 with a total whip lenght of 49cm.

As the elevated mount basically turns the entire antenna assembly into a vertical dipol I decided to simulate the thing in Mmangal.
Should be straight forward, diameter and length of the ground rod, wire length and diameter for the actual whip.
But with the same dimensions I have working fine on the car Mmangal tell me my antenna is totally out of whack.
Calculated SWR is well over the 15 and the antenna is not anywhere near resonant in the 470MHz band.
Mmangal is mainly for lower frequencies but shouldn't it work for the 70cm band as well?
If I design the antenna as a classic ground plane with 3 or 4 radiating elements sloped at 42° I still get nowhere with my whip length.
Since my SWR meter is correct and the antenna working really well I am wondering where my problem with Mmangal might be?
Anyone here using it for 470MHz or with some insight on how to get the correct values for a vertical dipol antenna with a rod for ground and wire for the whip?

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